This invention relates to explosive devices commonly referred to as hollow charges or shaped charges. These essentially comprise a symmetric explosive charge within which is formed a cavity lined by a lining material. When the explosive charge is detonated the liner, of metal in known devices, is subject to extremely high compressive loads which act to collapse and eject the liner material in the form of a high speed fluid jet, normally followed by a more slowly moving rigid slug. The charge and liner may be rotationally symmetric or non axi-symmetric, for example with a liner with a xe2x80x9cVxe2x80x9d cross section, used for cutting operations.
There are a number of industrial applications for shaped charge devices where rapid penetration effects are required in awkward and inaccessible places. An example is to initiate or increase the yield of oil and gas wells. In this case a number of charges are arranged to fire radially outwards at the base of the well. Upon detonation the shaped charge jets perforate the steel well casing, surrounding concrete grouting and then penetrate deeply into the oil/gas bearing rock, producing a series of discrete channels through which the oil and gas can flow into the well conduit. Another application is perforation and clearance of refractory bung at the base of a steel smelting crucible. The most extensive use, however, is in the military context against heavily protected targets such as tanks and shelters and for a wide range of battlefield engineering applications. In all these cases the shaped charges are designed and applied to exploit their penetration potential.
The present invention seeks to provide a shaped charge explosive device particularly suitable for use for avalanche control. However, the mechanism by which energy is distributed and imparted to the target medium by this invention offers potential for a number of alternative applications. The invention will be described in context with avalanche control applications first, followed by alternative applications.
Avalanches can present a serious danger to people and property when triggered in an uncontrolled manner, whether by a natural cause such as the weather conditions or unintentionally as a result of human activity such as skiing or climbing. It has therefore become an established practice in many mountainous areas to maintain a continuous programme of avalanche control using explosives to trigger a release. This practice of regularly triggering small controlled avalanches is intended to minimise the build up of snow in known start zones which, if left, would eventually release naturally and unexpectedly often cascading out of control. The current practices relevant to the present invention include the following.
Where avalanche start zones are inaccessible, an explosive charge can be delivered to the slope in the form of a projectile fired from a gun or mortar system where the projectile explodes on or shortly after impact. Short ranges (up to 3 km) can be covered by gas gun projector systems such as the nitrogen driven Avalauncher, used extensively in the US, Canada and Europe. Longer ranges demand high performance systems typical of military artillery and the 105 mm howitzer and 106mm recoilless rifle have been used in avalanche control operations for many years.
Fuzes in older military ammunition are designed to detonate upon impact, in soft snow, however, these fuzes tend to trigger well below the surface and quite probably not until the projectile strikes rock or firm ground. In fact, the ideal point of burst for avalanche release is several meters above the surface in proximity mode. However, with gun fired projectiles, this can only be achieved with an electronic proximity burst fuze. Since this type of fuze is both inhibitively expensive and notoriously unreliable against light, dispersed media such as snow, the performance of impact fuzing continues to be tolerated.
Most areas in ski resorts are accessible, including the mountain peaks, and this accessibility enables explosive charges to be delivered or placed by hand. The practice of positioning charges by hand is probably the most cost effective and extensively used method of avalanche control in many ski resorts, but carries with it obvious hazards in poor weather conditions. The hand charge is a relatively simple device consisting of a lightly cased (cardboard) explosive charge detonated by a length of capped pyrotechnic delay fuze. The fuze can be ignited and the charge thrown into a preferred position or the charge can be pre-positioned above the surface on a bamboo stick before the fuze is ignited.
It is acknowledged that various types of anti-tank ammunition, bearing shaped charge liners, have been fired into avalanche start zones in the past but this has been as a result of ammunition availability rather than an interest in the shaped charge effect. Results from this type of ordnance, designed specifically for high penetration into steel, has nevertheless been no different from standard artillery fragmenting shells because little of the jet energy can be dissipated into the snow pack.
The present invention seeks to provide an improved hollow charge explosive device for this and other applications.
Accordingly, the present invention provides a hollow charge explosive device including an explosive charge defining boundary walls of a cavity and including particulate material located forward of said boundary walls so as to be dispersible by said explosive charge when detonated.
The particulate material may be included in a liner lining the cavity or positioned elsewhere forward of the cavity, eg in a nacelle, or in both positions.
The particulate material, if present in a liner, is driven in the same way as that of a conventional shaped charge liner. However, in this case, the particulate medium forms into a highly energetic non-cohesive stream of particles, generally wider than that produced by a conventionally lined shaped charge. In this highly energised state, the low bulk density of the liner material and high surface area attributable to each particle of the liner material, together with the larger surface area of the jets cross section, facilitates an intimate and violent kinetically stimulated reaction with the target medium. Given a knowledge of the intended target material and its constitution, eg a snow slab, the liner material can be chosen to optimise the blast energy yield over and above that normally attributable to the explosive charge alone.
Conveniently, the liner may comprise an inner liner skin and an outer liner skin defining a space therebetween and the particulate material may be a loose powder contained in that space. In a one embodiment, the inner liner skin and outer liner skin are of a glass reinforced plastics material. The particulate material may be aluminium powder, particularly for use in avalanche control due to the potentially highly reactive nature of aluminium powder with water.
In an alternative embodiment, the particulate material may be embedded in an inert binder such as a plastics material, a was such as a paraffin was, or an adhesive matrix to aid manufacture, handling and assembly. The matrix material may also be conveniently chosen to make a net contribution to the reaction of the principal suspended particulate material.
Where a liner is not present, the high pressure and high temperature gaseous stream produced by the hollow cavity in the explosive focuses blast effects only along the axis of the charge. If a particulate material is located on the axis of the charge, typically in the nacelle, this material will be energised and dispersed by the high pressure and high temperature gases ejected from the cavity, thereby further enhancing the directed blast effects produced by the hollow cavity.
An explosive device assembly may be formed from two such explosive devices oriented such that the jets of liner formed on detonation of the charges are directed towards each other or away from each other.
When the jets are directed toward each other, the collision of the jets with each other provides an energetic response between the interacting jets. Two or more dissimilar liner materials may be provided in the explosive devices which when brought together in collision with each other and/or the target medium achieve an energetic response between associated interacting materials. This effect may also be further enchanced with additional particulate material located in the nacelle.
The devices may be gun fired, or otherwise hand thrown, or form part of a mechanically or chemically launched projectile.
An elongate support may be attached to the explosive charge body to aid hand positioning the device at the target.
The liner material may take any convenient form which can produce a shaped charge liner collapse mechanism, the so-called xe2x80x9cMunroe effectxe2x80x9d, and typically include conical liner configurations and hemispherical and hemispherical cap geometries.
A method of triggering an avalanche according to the present invention comprises positioning an explosive device or explosive device assembly of the present invention in a predetermined position relative to a snow or ice formation and detonating said explosive device or device assembly.